JP2883787B2 - Substrate for power semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a substrate for a power semiconductor device applied to a power transistor module and the like, in particular, a DBOC substrate (Direct Bonding of Copper) in which a copper plate is directly bonded to a ceramic substrate.
Substrate).

[0002]

2. Description of the Related Art First, FIG. 1 shows a structure of a power semiconductor device assembled using a DBOC substrate described above. In the drawing, 1 is a heat-dissipating metal base, 2 is a DBOC substrate in which copper plates 2b and 2c are bonded to both sides of a ceramic substrate 2a, 3 is a semiconductor chip, 4 is an external lead terminal, 5 is a bonding wire, 6 is a resin case, 7 is a terminal block, 8
Is a sealing resin, and 9 is a gel filler.

Here, a DBOC substrate 2 is formed by laminating a foil-like thin copper plate 2a, 2b on both surfaces of a ceramic substrate 2a such as alumina or aluminum nitride by a direct bond copper method (copper and trace oxygen). (A Cu-O eutectic liquid phase generated by the reaction is bonded as a bonding agent). A direct bonding is performed. After a circuit pattern is formed on the copper plate 2c on the main surface side, the semiconductor chip 3 is die-bonded thereto. Further, after assembling the circuit by bonding the wires 5, the DBOC substrate 2 is soldered on the heat dissipating metal base 1, and then the external lead terminals 4 are soldered and packaged.

[0004]

By the way, the aforementioned DB
When the OC substrate is used as a substrate of a power semiconductor device such as a power transistor module, there are the following problems. That is, in a semiconductor device such as a power transistor, a large amount of heat is generated in the semiconductor chip 3 due to the energizing operation, and this heat is transferred to the heat radiating metal base 1 via the DBOC substrate 2 and then radiated to the outside from the metal base. Is done. Therefore, as can be seen from this, the quality of the heat conductivity of the DBOC substrate is an important factor that affects the current capacity of the semiconductor device.

Incidentally, the DBOC substrate 2 has a relatively low thermal conductivity because it has a structure using the ceramic substrate 2a as an insulating base material. Here, a comparison of the thermal conductivity between alumina and aluminum nitride, which are conventionally used as the material of the ceramic substrate 2a, shows that alumina: 21 W / m · K aluminum nitride: 180 W / m · K Heat transfer is much better than alumina, but aluminum nitride has a higher material cost than alumina.

Accordingly, the inventors have attempted to reduce the heat transfer resistance of a ceramic substrate made of alumina, which has a low material cost, by reducing the thickness of the substrate as much as possible in order to enhance heat dissipation. However, when the thickness of the alumina ceramic substrate is reduced to, for example, about 0.3 mm, the actual strength of the substrate is reduced, and the thermal expansion of each material upon joining with the semiconductor chip (silicon) and the heat dissipation metal substrate (copper) is based on this. It has been found that cracks and cracks are frequently generated in the substrate due to the thermal stress applied due to the coefficient difference, and this causes insulation failure in the substrate. The thermal expansion coefficients of the respective materials are listed below.

Silicon (semiconductor chip): 4.0 × 10 −6 / ° C. Alumina: 7.5 × 10 −6 / ° C. Copper: 18.0 × 10 −6 / ° C. The present invention has been made in view of the above points. The ceramic substrate mainly made of alumina, which has a low material cost, improves the mechanical strength of the material, especially its flexibility, by improving its material composition, and reduces the heat dissipation as well as the thinness of the substrate itself. It is an object of the present invention to provide a power semiconductor device substrate capable of improving the above.

[0008]

SUMMARY OF THE INVENTION The object of the present invention is to provide a power semiconductor device substrate in which a copper plate is directly bonded to a ceramic substrate. 10-30
Achieved by constituting a high-temperature fired body in which any one of yttria, calcia, magnesia, and ceria is further added in a simple manner to the total amount of zirconia added by wt%. .

Further, as the zirconia added to the alumina of the ceramic substrate, stabilized zirconia containing any one of yttria, calcia, magnesia and ceria, or partially stabilized zirconia can be used.

[0010]

As described above, the ceramic substrate obtained by adding alumina to the main component and adding zirconia to the ceramic substrate and firing the same has a significantly higher mechanical strength, particularly bending strength, than the ceramic substrate of alumina alone. By selecting the amount of addition in the range of 10 to 30 wt%,
The mechanical strength, especially the flexibility, is improved without lowering the thermal conductivity of the ceramics substrate. Further, any one of yttria, calcia, magnesia, and ceria of 0.5 to 2 wt% with respect to the total amount of alumina and zirconia is used. By adding the two, the toughness of the zirconia crystal grains is improved while the firing temperature of the ceramic substrate is kept low. Thus, a DBOC substrate having excellent heat dissipation can be obtained as a substrate of a power semiconductor device by reducing the thickness of the substrate itself.

In addition, yttria, calcia, magnesia, and ceria are added alone, and one of them is added.
The same effect can be obtained by adding zirconia stabilized or partially stabilized with two additives to alumina.

[0012]

Embodiments of the present invention will be described below. First, zirconia and yttria are added to alumina and pulverized and mixed to a particle size of about 0.5 to 3 μm. Further, 8 wt% of polyvinyl butyral PVB as a binder, 50 wt% of a mixed solution of toluene and xylene as a solvent, and phthalic acid as a plasticizer Dioctyl DOP is added at 2 wt% to about 20
After kneading for a time, it was formed into a sheet by a doctor blade method to obtain a green sheet. Next, the green sheet is cut into a predetermined shape by press working,
The resultant was fired at 50 to 1650 ° C. to prepare a ceramic substrate having a thickness of 0.32 mm.

Further, in order to evaluate the mechanical properties of the ceramic substrate prepared by adding zirconia to alumina as described above, the amount of zirconia added was varied in the range of 0 wt% to 45 wt%. A sample piece prepared in the same manner as that described above was prepared, and a strength test was performed on this sample piece. FIG. 2 shows the results of the strength test. As can be seen from FIG. 2, by adding zirconia to alumina in an amount of 10 wt% or more, a ceramic substrate of alumina alone (the amount of zirconia added was 0 wt%) (strength of about 300 MP) was used.
The strength is higher than that of a), and the strength is about 350 to 40
It is recognized that it is improved to 0 MPa. However, no increase in strength was observed even when the amount of zirconia added was increased to 30 wt% or more.

On the other hand, when the thermal conductivity of these sample pieces was examined, the results as shown in FIG. 3 were obtained. That is, it was found that when the added amount of zirconia was up to about 30 wt%, the thermal conductivity was substantially the same as that of alumina alone, but when the added amount of zirconia exceeded 30 wt%, the thermal conductivity was lowered. Therefore, the material composition of the ceramic substrate is set to 70 to 90 wt% of alumina and 10 to 30 wt% of zirconia.
By selecting the above range, the mechanical strength is improved as compared with the ceramic substrate made of alumina alone, and the heat conductivity can be obtained as high as that of alumina.

A ceramic substrate sample (thickness: 0.32) was prepared with the addition amount of zirconia being 10 wt%.
mm, width 35 mm) and a sample piece of the same size made of alumina alone was subjected to a bending test at a fulcrum distance of 40 mm, and the maximum amount of deflection was examined. .30 mm,
It was confirmed that the zirconia-added sample piece could withstand a deflection of 0.45 mm.

The yttria added in addition to alumina and zirconia in the above material composition is added for the purpose of improving the toughness of sintering aids for alumina and zirconia and zirconia crystal grains. In this case, the addition amount of yttria is preferably set in the range of 0.5 to 2 wt%. That is, if the amount of yttria added is 0.5 wt% or less, the firing temperature of the substrate becomes 1650 ° C. or more, and it becomes difficult to manufacture the substrate. Further, when the addition amount is 2 wt% or more, a large variation occurs in the shrinkage ratio and strength of the ceramic substrate. The same effect can be obtained by adding calcia, magnesia, ceria, etc., as a single additive, in addition to yttria, and the above-mentioned yttria, calcia,
It has been confirmed that similar effects can be obtained by using stabilized or partially stabilized zirconia such as magnesia and ceria.

Next, on the above-mentioned zirconia-added alumina substrate (thickness 0.32 mm), tough pitch electrolytic copper having a thickness of 0.3 mm is superimposed on both front and back surfaces, and nitrogen at a temperature of 1050 to 1075 ° C. After heating in an atmosphere for 10 minutes to directly join the ceramic substrate and the copper plate to form a DBOC substrate, the semiconductor device of FIG. 1 was assembled using the DBOC substrate. As a comparative example, a similar semiconductor device was separately assembled using a DBOC substrate made of a ceramic substrate of alumina alone having a thickness of 0.63 mm and 0.32 mm, and mechanical deformation resistance was attached to these assemblies. The test and the intermittent current test were performed.

First, in the deformation resistance test, an external force was applied to the heat-dissipating metal base to forcibly deform it, and the amount of deformation until the ceramic substrate became defective in insulation was examined. As a result, the alumina substrate with a zirconia addition (plate thickness 0.32 mm) and the alumina substrate with a plate thickness of 0.63 mm showed almost the same resistance, but the alumina substrate with a plate thickness of 0.32 mm had a deformation amount of about half. It was found that this resulted in poor insulation. That is, by adding an appropriate amount of zirconia to alumina, it is possible to reduce the thickness of the ceramic substrate to about 1/2 in practical terms.

In the intermittent conduction test, the zirconia-added alumina substrate (thickness: 0.32 mm) was found to have a thickness of 0.63 mm.
mm, the temperature rise of the bonding block temperature of the semiconductor chip when the same collector loss is given as compared with the alumina single substrate of mm.
It was recognized that Tj was reduced by about 20%, and the intermittent current resistance was improved 2.5 to 3 times. As can be seen from the above, the ceramic substrate obtained by adding zirconia to alumina has excellent mechanical properties compared to the substrate of alumina alone, and is sufficiently used as a substrate for a semiconductor device having high heat dissipation due to the thinness of the substrate itself. Can be expected.

Although the thickness of the ceramic substrate is set to 0.32 mm in the above embodiment, a substrate having a thickness of 0.05 to 1 mm can be formed by a sheet forming method.
In addition, if a press molding method is used in which a binder such as polyvinyl alcohol PVA is added to the raw material powder, and the mixture is wet-mixed and then formed using a raw material dried and granulated by a spray dryer, a substrate having a thickness of 1 mm or more can be formed. It is possible.

[0021]

As described above, according to the present invention, as the ceramic substrate of the power semiconductor device, 70 to 90 wt% of alumina is added to the total amount of 10 to 30 wt% of zirconia added thereto. 0.5-2wt
% Of yttria, calcia, magnesia, and ceria,
The mechanical strength can be greatly enhanced without causing variations in shrinkage and strength as compared with a conventional alumina-only ceramic substrate. Therefore, it is possible to reduce the thickness of the ceramic substrate in practical use, thereby obtaining a DBOC substrate having high heat dissipation as a substrate for a power semiconductor device, and greatly contributing to downsizing of the power semiconductor device and increase in current capacity. it can.

[0022]

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a configuration of a semiconductor device assembled using a substrate according to the present invention.

FIG. 2 is a diagram showing the relationship between the amount of zirconia added to alumina and the bending strength of a substrate.

FIG. 3 is a diagram showing the relationship between the amount of zirconia added to alumina and the thermal conductivity of a substrate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heat dissipation metal base 2 DBOC substrate 2a Ceramics substrate 2b, 2c Copper plate 3 Semiconductor chip

──────────────────────────────────────────────────続 き Continuing on the front page (72) Katsumi Yamada 1-1-1, Tanabe-Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fuji Electric Co., Ltd. No. 1 Sumitomo Metal Ceramics Co., Ltd. (72) Inventor No ▲ Saki ▼ Toshio 2701 Iwakura, Ominecho, Mine-shi, Yamaguchi Pref. Sumitomo Metal Ceramics Co., Ltd. (72) Inventor Kazuhiko Teramura 2701-1 Iwakura, Sumitomo Metal Ceramics Co., Ltd. (56) References JP-A-3-149860 (JP, A) JP-A-4-21564 (JP, A) (58) Fields investigated (Int. Cl. ⁶⁾ H01L 23/14 C04B 35/00

Claims (2)

(57) [Claims] 1. A power semiconductor device substrate in which a copper plate is directly bonded to a ceramic substrate, wherein the ceramic substrate has a main component alumina of 70 to 90 wt% and a total amount of 10 to 30 wt% zirconia added thereto. Further, a substrate for a power semiconductor device characterized by comprising a fired body to which 0.5 to 2 wt% of any one of yttria, calcia, magnesia and ceria is simply added. 2. The substrate according to claim 1, wherein the zirconia added to the alumina is a stabilized zirconia containing any one of yttria, calcia, magnesia and ceria, or a partially stabilized zirconia. For power semiconductor devices.